Transparent mask and method for making the same

ABSTRACT

Transparent glass mask having a copper stain formed therein, the outer perimeters of which have a depth greater than the inner portion of the stain and a method for making the same in which a pattern of aluminum is formed on the glass and serves as a mask for driving the copper into the glass.

United States Patent [111 3,620,795

[72] Inventor William M. Kiba 2,904,432 9/ 1959 Ross 96/34 San Jose, Calif. 2,911,749 11/1959 Stookey... 96/34 [21] App1.No. 724,903 2,927,042 3/1960 Hall 1 17/38 [22] Filed Apr. 29, 1968 3,135,638 6/1964 Cheney 96/36.2 [45] Patented Nov. 16, 1971 OTHER R C S [73] Asslgnee g z ggg Metal on Glass Masks, George and Seaman, The Western unnyv Electric Engineer," April, 1967 (pp. 29- 30) Primary Examiner-Norman G. Torchin [54] TRANSPARENT MASK AND METHOD FOR Assistant Examiner-Judson R. Hightower MAKING HE AM Aztorney-Flehr, Hohbach, Test, Albritton & Herbert 8 Claims, 11 Drawing Figs.

[52] US. Cl 1 17/55, 11718.5,117/124, 96/36 [51] Int. Cl B4411 l/52 Field of Search ..96/34, 36.2, Transparent gIass mask having a Copper Stain 36, 38.3; 1l7/5.5, 8.5, 33 formed therein, the outer perimeters of which have a depth greater than the inner portion of the stain and a method for [56] Reerences cited making the same in which a pattern of aluminum is formed on UNITED STATES PATENTS the glass and serves as a mask for driving the copper into the 2,732,298 1/1956 Stookey 96/34 glass.

TRANSPARENT MASK AND METHOD FOR MAKING THE SAL/[E BACKGROUND OF THE INVENTION In application Ser. No. 666,907, filed Sept. 11, 1967, there is disclosed a transparent mask and method for making the same. It has been found that in utilizing the method disclosed therein that there is some tendency for the edges of the pattern formed by the stain in the glass to bleed into the surrounding areas to thereby provide a gradient in color in the color centers which provides an undesirable density change at the outer margin of the pattern. It has been found that this characteristic is particularly objectionable where the mask is used in conjunction with long exposure times in the processing of wafers for the making of semiconductor devices. For example, it has been found that a change of 2 or 3 seconds in the exposure time of a wafer by the use of such a mask may give a change of approximately 1 micron to 2 microns in line width in the pattern which makes it difficult or impossible to attain the desired dimensional stability required for the production of certain semiconductor devices. There is, therefore, a need for a new and improved transparent mask and a method for making the same.

SUMMARY OF THE INVENTION AND OBJECTS The transparent mask consists of a glass substrate having a glass stain formed therein adjacent one surface and which carries a desired pattern. The pattern of the stain is characterized in that lines in the pattern have a line width of less than 4 microns and in that the outer margins of the pattern have a depth which is greater than the inner portions of the pattern. In forming the transparent mask, a layer of material and a layer of copper are deposited on the surface of a glass substrate. Either the layer of material or the copper can be deposited first. Openings are formed in the layer which is deposited first to expose portions of the surface of the substrate to provide a pattern in the material. The other layer is then deposited over the first layer and into the openings exposing the same surface of the glass substrate. The copper is then caused to be driven into the exposed portions of the glass surface by using the material as a mask for the copper. Thereafter, the excess copper and the material which is used for forming the mask for the copper are removed to provide the transparent mask which has a stain therein of the desired pattern. The copper in the glass is then transformed into elemental copper having the desired opacity to ultraviolet light. If the same patterns are utilized, one arrangement of the layers will produce a positive stain in the glass substrate and the other will produce a negative stain in the glass substrate.

In general, it is an object of the present invention to provide a transparent mask having a pattern with sharp well-defined edges and narrow well-defined lines.

Another object of the invention is to provide a mask of the above character in which the outer margins of the pattern have a depth which is greater than the inner portions of the pattern.

Another object of the invention is to provide a method for making a transparent mask in which one of the two layers utilized serves as a mask for driving the copper that forms one of the layers into the glass substrate.

Another object of the invention is to provide a method of the above character in which the copper is driven in by the use of relatively high voltage.

Another object of the invention is to provide a method of the above character in which the pattern is formed in the aluminum layer.

Another object of the invention is to provide a method of the above character in which the pattern is formed in the copper layer.

Additional objects and features of the invention will appear from the following description in which the preferred embodiment is set forth in detail in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross-sectional view of a glass substrate utilized in the invention.

FIG. 2 is a cross-sectional view showing a layer of aluminum deposited on the glass substrate.

FIG. 3 is a cross-sectional view showing a layer of photoresist disposed on top of the layer of aluminum carried by the substrate.

FIG. 4 is a cross-sectional view showing the photoresist after the unexposed portions have been removed.

FIG. 5 is a cross-sectional view showing the exposed aluminum etched away.

FIG. 6 is a cross-sectional view showing the substrate with the aluminum pattern thereon with the photoresist etched away.

FIG. 7 is a cross-sectional view showing a layer of copper deposited over the aluminum pattern on the substrate.

FIG. 8 is a cross-sectional view showing schematically the apparatus utilized for driving the copper into the glass substrate.

FIG. 9 is a cross-sectional view showing the glass substrate with the stain formed therein to provide a pattern.

FIG. 10 is a greatly enlarged cross-sectional view of a portion of the substrate shown in FIG. 10 showing the manner in which the outer edge of the stain has a greater depth than the inner portions of the stain.

FIG. 11 is a cross-sectional view of a glass substrate which has a stain formed therein by another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The transparent mask 10 comprising the present invention is made starting with a substrate 1 1 formed of a glass which is substantially transparent to visible light and to ultraviolet light. As pointed out in copending application Ser. No. 666,907, filed Sept. 1 I, l967, the glass which is normally used as the substrate 11 is characterized in that it contains at least some boron or sodium. The glass also should be relatively inexpensive and should have good optical characteristics particularly in the ultraviolet region. One glass found to have these desirable characteristics is Corning 7740 glass which is a borosilicate glass having an annealing point of 650 C., a softening point of 820 C., and a working point of 1220" C. Corning 7740 glass has approximately percent transmission at 350 millimicrons which can be considered to be the start of the ultraviolet region which is of interest with respect to the present invention.

The substrate 11 can have many desired dimensions. For example, one type of substrate found to be particularly satisfactory is 2 inches by 2 inches and has a thickness of one-sixteenth of an inch. The substrate is generally provided with two generally parallel planar surfaces 12 and 13. At least one of the surfaces, such as surface 12, should be optically polished to remove all defects. The surface 12 is then cleaned in a conventional manner such as by dipping the same in heated chromic acid for 5 or l0 minutes and then rinsing the same thoroughly in deionized water. Thereafter, the substrate 11 is dipped in acetone and placed in an oven at 180 F. for approximately 30 minutes to dry the same and to provide the dry, clean substrate 11 which is shown in FIG. 1.

A thin aluminum layer or film 14 is evaporated onto the surface 12. Care is taken so that the aluminum is not evaporated over the edge of the substrate 11 so that the outer margin of the upper surface 12 is exposed as shown in FIG. 2. The aluminum can be deposited in any suitable manner such as by evaporation. The aluminum is evaporated to a suitable depth such as 500 Angstroms. However, if desired, the thickness can vary considerably from this dimension.

After the aluminum has been applied as shown in FIG. 2, the substrate with the aluminum film or layer thereon is placed upon a photoresist spinner revolving at 3,600 rpm. for approximately 30 seconds and a layer 16 of photoresist is applied onto the top of the substrate 11 and over the aluminum layer or film 14 as shown in FIG. 3. A suitable photoresist is identified as AZl350. It has been found that in the depositing of this photoresist, it is desirable to filter the same through a 1 micron filter to remove any partially polymerized particles. After the photoresist has been spun to uniform thickness onto the substrate 11 and the aluminum layer 14 and permitted to dry, it is exposed to ultraviolet light through an emulsion mask (not shown). The emulsion mask has the pattern to be formed in the aluminum layer 14. The photoresist is developed in AZI350 photoresist developer and then rinsed in deionized water. Openings or windows 17 are formed in the photoresist to form an image in the photoresist which corresponds to the image carried by the emulsion mask.

As shown in FIG. 5, the portions of the aluminum layer 14 which are exposed through the openings or windows 17 are etched away by sodium hydroxide (NaOI-I) or phosphoric acid, although it has been found that the use of phosphoric acid is preferable. The etching is continued until all the aluminum is removed down to the surface 12 of the glass substrate 11 to provide windows or openings 18 which are in alignment with the windows 17 provided in the photoresist and thus form a pattern or image in the aluminum which corresponds to the pattern carried by the emulsion mask.

As shown in FIG. 6, the remaining AZl350 photoresist is removed by the use of an acetone rinse followed by a rinse of deionized water. The substrate 11 is then blown dry to provide a substrate 11 which has an aluminum layer thereon which carries the image or pattern of the emulsion mask.

Next as shown in FIG. 7, a layer of copper is formed on top of the substrate 11 and in so doing is deposited directly upon the exposed portions of the optical surface 12 of the substrate 11 through the windows 18 and also over the portions of the aluminum layer 14 which remain. Precautions are taken so that during the evaporation of the copper layer 19, copper is not deposited adjacent the outer margin of the planar surface 12. The layer 19 is formed in any suitable manner such as deposition of the same in an evaporator. The copper is deposited to a suitable depth such as 1500 Angstroms. The

thickness is somewhat dependent upon the density of the stain' desired. Thus, if a lighter shade of stain is desired, a thinner layer of copper is deposited, whereas if a darker shade of the stain is required, a thicker layer of copper is deposited. It is believed that a minimum thickness, however, would be approximately 500 Angstroms. The maximum thickness is not critical as long as the edge definition is retained.

The laminated structure 20 shown in FIG. 7 comprising the substrate 11, the aluminum layer 14 and the copper overlayer 19 is placed in apparatus of the type shown schematically in FIG. 12 which is utilized for causing the copper to be driven into the glass substrate. The use of such means is substantially conventional and is typically described as apparatus for causing field aided diffusion. The apparatus includes a pair of electrodes 21 and 22 formed of a suitable conducting material such as aluminum and having a size which is at least slightly greater than the surfaces 12 and 13 of the substrate 11. The lower surface of the electrode 21 and the upper surface of the electrode 22 have been carefully polished to provide very flat surfaces. Means is provided for heating the electrodes 21 and 22 and consists of a heater block 23 which carries a pair of heaters 24 for maintaining the heater block 23 at a predetermined temperature as, for example, from 320 to 820 C. It has been found that a temperature of 345 is one of the most advantageous. Means is provided for isolating the heater block from the lowermost electrode 22 and consists of a sheet of glass 26 disposed between the heater block 23 and the electrode 22.

As can be seen, the electrode 22 makes contact with the bottom surface of the substrate 22, whereas the upper plate makes contact with the copper layer 19. Since a high voltage DC is applied to the electrodes 21 and 22, neither the aluminum nor copper is deposited over the edges of the substrate 1 1 so as to prevent shorting out the electrodes 21 and 22.

In driving the copper into the glass substrate, it has been found that it is desirable to allow a suitable period of time for the substrate 11 to come up to the temperature of the heater block as, for example, approximately 3 minutes. As soon as the substrate 11 has been brought up to the desired temperature as, for example, 345 C., the high voltage DC is applied to the substrates 21 and 22 as, for example, 1500 volts for a period of approximately 10 seconds. During the application of this high voltage, there is some corona discharge within the glass substrate 11 and around the electrodes 21 and 22. By the utilization of such voltages at such a temperature, the copper is driven into the surface 12 of the glass substrate wherever the surface is exposed to the copper to the desired depth to stain the glass.

After the power has been turned off, the laminate structure 20 comprising the substrate 11, the aluminum layer 14 and the copper layer 19 are removed. The copper which remains on the surface 12 is excess and is removed in a suitable manner such as by dipping the laminate structure in chromic acid for a suitable period of time such as 5 or 6 minutes. The chromic acid removes the excess copper and any copper oxide that may have been formed. The substrate 11 is then rinsed with water and placed in a sodium hydroxide solution to remove the aluminum which remains on the surface 12. The substrate 11 is then rinsed in deionized water and blown dry. At this point, the substrate 11 has the appearance shown in FIG. 9 in which all that remains is the glass substrate 11 with areas of stain 31 disposed in the surface 12 with an image corresponding to the pattern formed by the emulsion mask. The stain appears to have a very light amber color and under inspection under a microscope, it is possible to see the faint outlines of the image which it is desired to form.

The substrate is then taken and placed in a diffusion furnace operating at a temperature of approximately 600 C. and forming gas comprising 20 percent hydrogen and percent nitrogen is passed over over the substrate at a flow rate of approximately 2 liters per minute. The substrate 11 is permitted to remain in this forming gas stream in the hot zone of the diffusion furnace for a period of time ranging from l5 to 30 minutes. The hydrogen of the forming gas diffuses into the surface 12 and reduces any copper that has been driven into the glass substrate. It is believed that during the process shown in FIG. 9 that the copper enters the glass substrate as a copper oxide and that when the hydrogen from the forming gas diffuses into the glass substrate, the forming gas reduces the copper oxide to elemental copper to provide the red color center which gives the stain its characteristic red color. After the substrate has been heated for a sufiicient time in the diffusion furnace, it is removed from the furnace and permitted to cool. The mask is now ready for use for the formation of semiconductor devices as described in Pat. application Ser. No. 666,907, filed Sept. 11, 1967.

It has been found that the depth of the stain can be increased either by increasing the voltage which is applied to the electrodes 21 and 22, or by increasing the time of application of the voltage, or both. However, it has been found that there is slightly more lateral spreading with each increase in voltage or increase in time, or both.

It has been found that the transparent mask which is formed by the above method has many superior qualities. In particular, it has been found that the image or pattern formed has very sharp edges or, in other words, the edge definition is excellent. In addition, it has been found to be possible to provide patterns or images having well defined lines having a width of less than 4 microns. Typically, lines having a width of 3.4 microns have been formed.

It is believed that one of the most important steps in the method set forth above which makes possible the formation of such a mask is the use of a mask of a different material on the substrate itself and through which the copper is driven into the glass. It has been found that aluminum is particularly advantageous for this mask for the copper because it adheres very well to the surface of the glass substrate 11. In addition,

the aluminum mask is desirable because it does not react with the glass under high voltage and does not form color centers. Also, there are many etches available for working with aluminum and, therefore, it is very easy to provide excellent images in aluminum so that the aluminum can be used for forming sharp images in the copper stain. Since copper is deposited over the aluminum, there is no danger of undercutting of the copper during the diffusion step.

In examining stains which have been made in accordance with the present method and cross-sectioning a stained area, it has been found that the stain which appears adjacent the outer margins of the pattern penetrate to a depth which is greater than the depth of the other portions of the stain. Thus, for example, as shown in FIG. 10, it has been found that the stain has downwardly depending, inwardly extending portions 31a adjacent the outer perimeter of the stain which have a thickness which is substantially greater than the interior portion 31b of the stain. Thus, for example, the portion 31a had a depth of 9.6 microns, whereas the portion 3117 had a depth of 5,5 microns. It is believed that this characteristic of the stain is caused by the higher field strengths of the edges of the pattern in the aluminum layer. This increased thickness aids in more definitely defining the edges of the pattern or image formed by the stain.

It is also believed that this excellent edge definition is obtained because the plates 21 and 22 are provided with finely polished surfaces which are exactly parallel to each other to provide an electric field between the plates which is perpendicular to the plates and, therefore, cause the copper to be driven straight into the glass substrate 11 whereby the image formed on the glass is identical to that formed in the aluminum layer 14. Good contact is maintained between the copper and the upper electrode 21 and it is important that there be sufficient copper deposited in the layer 19 so that there is copper remaining after the voltage is removed from the electrodes 21 and 22 so that at all times during the process there is good contact between the electrode 21 and the copper layer 19.

Another embodiment of the transparent mask and method for making same is shown in FIG. 11. In many respects the method is very similar to that shown in FIGS. 1-9 and, therefore, it will not be shown in detail in the drawings. The steps which are described in connection with the previous embodiment for preparation of the body 11 are performed. Thereafter, as shown in FIG. 2, a thin layer or film 14 is evaporated onto the surface 12. However, instead of aluminum being used for this layer, copper is utilized which is deposited to a thickness of approximately 1500 Angstroms. A layer 16 of photoresist is then placed over the aluminum and then exposed through an emulsion mask to provide the desired pattern in the photoresist. Windows 18 are formed in the photoresist layer 16 and the exposed copper is etched away by a suitable etch such as a mixture of ferric chloride, hydrochloric acid and water. The etching is continued until the surface 12 of the glass substrate appears. The remaining photoresist is then removed as shown in FIG. 6.

Thereafter, as shown in FIG. 7, in place of the layer of copper, there is formed a layer of aluminum 19 which is deposited directly upon the exposed surfaces 12 in the windows l8 and over the top of the copper portions which remain of the copper layer 14. The aluminum is deposited to a suitable thickness such as approximately 500 Angstroms. The laminated structure 20 shown in FIG. 7, formed with portions of the copper layer 14 overlaid with the aluminum layer 19, is placed in apparatus of the type shown in FIG. 8 to drive the copper into the substrate to the desired depth in generally the same manner as described in conjunction with the previous embodiment. The substrate 20 is then removed from the apparatus and the aluminum and other metals and oxides are removed by a solution of phosphoric acid, nitric acid and water.

The substrate is then taken and baked and placed in a diffusion furnace with a forming gas to convert the copper which has entered the glass substrate into elemental copper as hereinbefore described.

The completed mask then has an appearance such as that as shown in FIG. 11 and which is very similar to that shown in FIG. 9 with the exception that the stained areas in FIG. 1 l are the negative of the stained areas in FIG. 9. Thus, when the aluminum layer is deposited first and a negative emulsion mask is used, a positive stain is provided in the glass substrate. Conversely, when the copper is deposited first and a negative emulsion mask is used, a negative stain is provided in the glass substrate. It, therefore, can be seen that either a negative or positive stain can be provided merely by reversing the order in which the layers are deposited.

In the latter embodiment it has been found that the aluminum still serves as a mask and prevents any substantial spreading of the copper during the driving of the copper into the substrate. Tolerances substantially as good as those which could be obtained by the method hereinbefore described can be obtained.

It is apparent from the foregoing that there has been provided a new and improved transparent mask in which the copper stain forms an integral part of the glass substrate. It has a long life and is relatively scratch resistant. The images are patterned in the mask and can only be destroyed if the substrate is broken. The copper stain is substantially transparent to visible light but is substantially opaque to ultraviolet light. The mask has excellent edge definition making it possible to utilize the same with long exposure times without appreciably affecting the dimensions of the semiconductor device which is being formed with the mask. In other words, the edges are very sharp and there is no tapering off of the opacity of the mask.

Iclaim:

l. A method for making a transparent mask for use in making a pattern on a layer of photoresist carried by a body comprising forming an optically polished planar surface on a glass substrate, forming a first layer of a first material, providing openings in the first layer of material to expose portions of the polished surface of the substrate to form a predetermined pattern on said optically polished surface, forming a second layer of a second material over said exposed portions of the optically polished surface and over the first layer of material, one of said first and second materials being essentially copper and the other of said first and second materials being essentially aluminum, causing the copper in contact with the optically polished surface of the glass to be driven into the glass, removing the excess copper from the optically polished surface and removing the remaining portion of the aluminum from the 0ptically polished surface so that the optically polished surface is substantially planar with the stain being disposed in the substrate below the optically polished surface.

2. A method as in claim 1 together with the step of reducing the copper which has been driven into the glass substrate into an elemental copper by exposing the glass substrate to a reducing gas at an elevated temperature.

3. A method as in claim 1 wherein the aluminum is deposited to a thickness of approximately 500 Angstroms and wherein the copper is deposited to a thickness of greater than 500 Angstroms.

4. A method as in claim 1 wherein the step of causing the copper to be driven into the glass includes the steps of first heating the substrate and carrying the copper to an elevated temperature and subjecting the copper to a strong electric field to aid in the diffusion of the copper into the glass substrate.

5. A method as in claim 4 wherein the electric field has lines which are substantially perpendicular to the layer of copper carried by the substrate so that the copper is driven straight into the substrate.

6. A method as in claim 4 wherein the substrate is heated to a temperature of approximately 345 C. and wherein said substrate is subjected to a voltage of approximately 1500 volts for a period of approximately 10 seconds.

7. A method as in claim 1 wherein a layer of aluminum is deposited first and wherein the openings are formed in the aluminum.

8. A method as in claim 1 wherein a layer of copper is deposited first and wherein the openings are formed in the copper. 

2. A method as in claim 1 together with the step of reducing the copper which has been driven into the glass substrate into an elemental copper by exposing the glass substrate to a reducing gas at an elevated temperature.
 3. A method as in claim 1 wherein the aluminum is deposited to a thickness of approximately 500 Angstroms and wherein the copper is deposited to a thickness of greater than 500 Angstroms.
 4. A method as in claim 1 wherein the step of causing the copper to be driven into the glass includes the steps of first heating the substrate and carrying the copper to an elevated temperature and subjecting the copper to a strong electric field to aid in the diffusion of the copper into the glass substrate.
 5. A method as in claim 4 wherein the electric field has lines which are substantially perpendicular to the layer of copper carried by the substrate so that the copper is driven straight into the substrate.
 6. A method as in claim 4 wherein the substrate is heated to a temperature of approximately 345* C. and wherein said substrate is subjected to a voltage of approximately 1500 volts for a period of approximately 10 seconds.
 7. A method as in claim 1 wherein a layer of aluminum is deposited first and wherein the openings are formed in the aluminum.
 8. A method as in claim 1 wherein a layer of copper is deposited first and wherein the openings are formed in the copper. 